Local oxidation of silicon (LOCOS) is the most commonly used isolation technology for silicon integrated circuits. Unfortunately, LOCOS has an inherently large field oxide encroachment that precludes it from being used in advanced integrated circuits requiring high device packing densities. In a standard LOCOS process a thin layer of pad oxide is thermally grown on the surface of a silicon wafer. A silicon nitride layer is then deposited onto the pad oxide layer. The silicon nitride layer is then photolithographically patterned and etched to define active regions and isolation regions. Field oxide is then grown in the isolation regions while the active regions, which are masked by the patterned silicon nitride layer, are protected from the oxidation process. After field oxide growth, however, the area of the resulting active region is smaller than the actual intended area, as defined by the patterned silicon nitride layer. This occurs because oxygen laterally diffuses through the pad oxide layer, underneath the patterned silicon nitride mask, and reacts with the underlying silicon surface. Therefore, field oxide is formed not only in the isolation regions, but it also encroaches into the adjacent active regions. As a result, scaling of active area dimensions is limited and therefore integrated circuits with high device packing densities cannot be achieved with standard LOCOS isolation.
In order to reduce field oxide encroachment, several LOCOS-like isolation techniques have been proposed. In one approach the pad oxide thickness is reduced and a layer of polysilicon is inserted between the pad oxide and the overlying silicon nitride. The polysilicon absorbs stress from the silicon nitride, thus allowing the silicon nitride film thickness to be increased and the pad oxide thickness to be reduced in order to minimize field oxide encroachment. Additionally, during field oxidation the exposed edge portions of the polysilicon layer become oxidized, thus consuming some of the oxidant species that would otherwise oxidize the substrate. After field oxidation the silicon nitride and polysilicon stack are then removed using wet or dry etching techniques. Unfortunately, the etch selectivity achieved between single crystal silicon and polysilicon is approximately 1 to 1 with these etching techniques. Therefore, during the polysilicon removal process if there are pinholes or defects present in the underlying pad oxide layer, then the underlying silicon substrate in the active region will be etched. The resulting damage adversely affects the functionality and reliability of semiconductor devices subsequently fabricated on these active regions. Furthermore, with this technique further reductions in field oxide encroachment, through increased thinning of the pad oxide layer, are limited due to the aforementioned problems associated with pinholes or defects.
In a second approach, the pad oxide layer lying underneath the silicon nitride oxidation mask is undercut to form a cavity. The cavity is then filled using a conformal layer of polysilicon. During field oxidation, the polysilicon filled cavity acts as a sacrificial layer by consuming oxidant species, and thus it inhibits the transportation of oxygen to the silicon surface underlying the edge of the silicon nitride oxidation mask. Unfortunately, after field oxidation unoxidized polysilicon may be left in the cavity region, which must be subsequently removed. Similarly, removal of the unoxidized polysilicon can result in the underlying active region being damaged, as discussed above. Accordingly, a need exists for an isolation process that reduces field oxide encroachment, but at the same time does not adversely effect the active regions.